// Copyright 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_MESSAGE_LOOP_INCOMING_TASK_QUEUE_H_
#define BASE_MESSAGE_LOOP_INCOMING_TASK_QUEUE_H_

#include "base/base_export.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/pending_task.h"
#include "base/synchronization/lock.h"
#include "base/synchronization/read_write_lock.h"
#include "base/time/time.h"

namespace base {

class MessageLoop;
class WaitableEvent;

namespace internal {

    // Implements a queue of tasks posted to the message loop running on the current
    // thread. This class takes care of synchronizing posting tasks from different
    // threads and together with MessageLoop ensures clean shutdown.
    class BASE_EXPORT IncomingTaskQueue
        : public RefCountedThreadSafe<IncomingTaskQueue> {
    public:
        explicit IncomingTaskQueue(MessageLoop* message_loop);

        // Appends a task to the incoming queue. Posting of all tasks is routed though
        // AddToIncomingQueue() or TryAddToIncomingQueue() to make sure that posting
        // task is properly synchronized between different threads.
        //
        // Returns true if the task was successfully added to the queue, otherwise
        // returns false. In all cases, the ownership of |task| is transferred to the
        // called method.
        bool AddToIncomingQueue(const tracked_objects::Location& from_here,
            const Closure& task,
            TimeDelta delay,
            bool nestable);

        // Returns true if the queue contains tasks that require higher than default
        // timer resolution. Currently only needed for Windows.
        bool HasHighResolutionTasks();

        // Returns true if the message loop is "idle". Provided for testing.
        bool IsIdleForTesting();

        // Loads tasks from the |incoming_queue_| into |*work_queue|. Must be called
        // from the thread that is running the loop. Returns the number of tasks that
        // require high resolution timers.
        int ReloadWorkQueue(TaskQueue* work_queue);

        // Disconnects |this| from the parent message loop.
        void WillDestroyCurrentMessageLoop();

        // This should be called when the message loop becomes ready for
        // scheduling work.
        void StartScheduling();

    private:
        friend class RefCountedThreadSafe<IncomingTaskQueue>;
        virtual ~IncomingTaskQueue();

        // Adds a task to |incoming_queue_|. The caller retains ownership of
        // |pending_task|, but this function will reset the value of
        // |pending_task->task|. This is needed to ensure that the posting call stack
        // does not retain |pending_task->task| beyond this function call.
        bool PostPendingTask(PendingTask* pending_task);

        // Wakes up the message loop and schedules work.
        void ScheduleWork();

        // Number of tasks that require high resolution timing. This value is kept
        // so that ReloadWorkQueue() completes in constant time.
        int high_res_task_count_;

        // The lock that protects access to the members of this class, except
        // |message_loop_|.
        base::Lock incoming_queue_lock_;

        // Lock that protects |message_loop_| to prevent it from being deleted while a
        // task is being posted.
        base::subtle::ReadWriteLock message_loop_lock_;

        // An incoming queue of tasks that are acquired under a mutex for processing
        // on this instance's thread. These tasks have not yet been been pushed to
        // |message_loop_|.
        TaskQueue incoming_queue_;

        // Points to the message loop that owns |this|.
        MessageLoop* message_loop_;

        // The next sequence number to use for delayed tasks.
        int next_sequence_num_;

        // True if our message loop has already been scheduled and does not need to be
        // scheduled again until an empty reload occurs.
        bool message_loop_scheduled_;

        // True if we always need to call ScheduleWork when receiving a new task, even
        // if the incoming queue was not empty.
        const bool always_schedule_work_;

        // False until StartScheduling() is called.
        bool is_ready_for_scheduling_;

        DISALLOW_COPY_AND_ASSIGN(IncomingTaskQueue);
    };

} // namespace internal
} // namespace base

#endif // BASE_MESSAGE_LOOP_INCOMING_TASK_QUEUE_H_
